1. Technical Field
The present invention relates generally to acoustics, and more particularly to an improved method and apparatus to reduce frequency response errors in small room acoustics.
2. Background Art
One challenge of small room acoustics is reducing frequency response errors introduced by standing waves, also known as “room modes”. There are many references on room modes in the literature, see, for example, P. M. Morse, Vibration and Sound, (McGraw Hill, N.Y. 1948), p. 313, 418; P. M. Morse and K. U. Ingard, Theoretical Acoustics, (McGraw Hill, N.Y., 1968), p. 576-598; J. Borwick (ed.), Loudspeaker and Headphone Handbook, (McGraw Hill, N.Y., 1968), ch. 7; and T. Welti, “How Many Subwoofers Are Enough”, presented at the AES112th Convention, Munich, Germany, 2002 May 10-13. Usually narrow in bandwidth, these frequency response errors can be up to 40 decibels in magnitude and are spatially variable within the listening environment. To achieve an accurate frequency response and consistent sound quality at all listener locations, the room modes can be controlled through absorption.
Sound Absorption Methods: Techniques for attenuating sound waves are resistive absorption, resonant absorption, and active cancellation.
Resistive Absorption: Resistive absorption reduces sound energy by dissipating it as heat. Resistive absorption is maximized where air particles exhibit maximum velocity and is minimized where they exhibit maximum pressure. High-density fiberglass insulation, mineral wool, open cell foam, and heavy velour drapes are examples of resistive absorbers. Deficiencies of resistive absorption are its broad frequency range of absorption (narrow band absorption is preferable for treating room modes, so non-modal frequencies are unaffected), and that it requires significant depth and area of material in the room to be effective at the low frequencies typical of room modes.
Because room boundaries are the most common locations for acoustical treatments, resistive absorption is not an effective method of standing wave absorption. A standing wave exhibits a pressure maximum and a velocity minimum at the room boundary. In order for a resistive absorber to be effective in absorbing a room mode standing wave, it must be located at least a quarter wavelength distance (relative to the frequency being absorbed) in from the room boundary. A standing wave exhibits a velocity maximum at its quarter wavelength. It is generally not practical to place acoustical treatments more than a few inches inward in the room from the room's boundary—low frequencies typical of room modes would potentially require treatments to be suspended one to three feet inward from the room's boundary.
Resistive absorption is more commonly used to shorten reverberation decay, eliminate flutter echoes, and to attenuate detrimental mid and high frequency reflections in the listening environment.
Resonant Absorption: Resonant absorption reduces sound energy by establishing a sympathetic resonance with the sound wave and applying a damping force to the resonant oscillation. Damping can be achieved by various methods. Resonant absorption is maximized where air particles exhibit maximum pressure and is minimized where they exhibit maximum velocity. Therefore, this method is ideally suited to applications at room boundaries, where pressure is maximized.
One type of resonant absorber is a tympanic diaphragm. Like a drum, the diaphragm is stretched over an airtight, enclosed chamber. A diaphragm absorber is tuned to the modal frequency and positioned at a pressure maximum (room boundary). Diaphragm mass density, enclosure air depth, and diaphragm tension establish the resonant frequency. When sound waves strike the diaphragm's surface, it resonates sympathetically. The diaphragm's tympanic flexing dissipates sound energy as heat and causes damping of the resonance. Although constructing accurately tuned diaphragm absorbers is difficult, it is possible to achieve a narrow frequency range of absorption (avoiding attenuation of adjacent non-modal frequencies). Adding resistive absorption material inside the air cavity of the tympanic absorber broadens the frequency range of absorption yet reduces the magnitude of absorption. Construction guidelines and background on tympanic diaphragm absorbers are common in the literature (see, e.g., A. Everest, Master Handbook Of Acoustics, McGraw Hill, NY, 2001, p. 205, p. 215-218).
A second resonant absorber is a pistonic diaphragm. The pistonic diaphragm is a rigid planar membrane with minimal tympanic flexing character. The goal with a pistonic membrane absorber is to excite the membrane in a purely perpendicular motion relative to the wave front (not to excite the complex drum-like flexing of tympanic resonant absorbers). The membrane is attached to mechanical springs that impose oscillation damping. As the membrane oscillates sympathetically in a pistonic motion perpendicular to the wave front, spring compression and expansion dissipate sound energy as heat. The membrane mass and spring constant determine the resonant frequency. Pistonic diaphragm absorbers are easily tuned and possess a narrow frequency range of absorption. The magnitude of absorption of pistonic diaphragms depends on the damping character of the springs employed in the design.
HO Cinema (BP 15, 36, rue Marcel-Deneux, 60780 Viller-St-Paul, France) makes one available form of a combination pistonic and tympanic diaphragm absorber. HO Cinema's absorber uses rubber spring strips which possess a significant resonant damping character. Sheets of drywall are attached to a system of tracks which sandwich the rubber springs between the drywall and the wall framing. As this system is a combination of pistonic and tympanic resonance, its bandwidth of absorption is broader than other absorption methods.
One deficiency of tympanic and pistonic diaphragm absorbers is that they can reradiate the sound energy of the standing wave into the room at a later time. The delayed reradiation of the sound energy is pyschoacoustically undesirable in a listening environment. Similar to echoes in a reverberant space, delayed radiation of bass energy in a modal environment seriously distracts from the desired effect of the program material (see Everest, supra).
Another resonant absorber is a Helmholtz cavity. A Helmholtz cavity is an enclosed chamber attached to an open cylindrical tube. The chamber's air volume and the tube length and diameter determine the resonant frequency. The tube opening is located at a pressure maximum. When sound waves encounter the tube opening, the cavity resonates sympathetically (similar to blowing on the mouth of a wine bottle). As the air in the tube and cavity compresses and expands, sound energy is dissipated by friction imposed by the column of air moving in and out of the tube. A challenge with Helmholtz absorbers is that the chamber must be very large to achieve low frequency resonance (in the region of frequency relevant to small room acoustics). Another limitation is the absorption coefficient produced by the resonance is considered to be applicable only over the tube opening area; numerous absorbers may be required to attenuate room modes (see Everest, supra). Two benefits of Helmholtz absorbers are that they are easily tuned and possess a narrow frequency range of absorption.
An example of an application of Helmholtz cavity absorbers exists today built into the ceiling of the Royal Festival Hall in London, England, designed by the architecture team of Sir Robert Mathew and Dr. Leslie Martin in 1949 (see B. Shield, The Acoustics Of The Royal Festival Hall, South Bank University, London, http://www.ioa.org.uk/articles/Rfh/rfh1.html). The Helmholtz cavities were tuned in the mid-frequency band (not for low frequency correction of standing waves) to reduce mid-frequency reverberation the hall. It was determined after construction however that too much attenuation had been achieved and some of the cavities were later filled. For more background on Helmholtz absorbers consult Everest, supra, and H. Olson, Acoustical Engineering, (D. Van Nostrand Co., Princeton, N.J., 1957), p. 508.
Active Cancellation: Another approach for attenuating room modes may be active cancellation. Active cancellation would inject a signal into the room at equal amplitude and opposite polarity as the standing wave, resulting in cancellation. An example of active sound cancellation has been produced by Sound Physics Labs in its airport runway noise suppression experiments using high-output subwoofer arrays in conjunction with phase-locked loop detectors (see C. Hobbs, Servodrive And Sound Physics Labs Speakers Reproduce Jet Engine SPLs For Noise Mitigation Research, http://www.ProSoundWeb.com/news/news01/servonoise.html). Similar research has also been conducted at Virginia Tech University on aircraft cabin noise suppression methods (see C. Fuller and A. Von Flotow, Active Control Of Sound And Vibration. “IEEE Control Systems Journal, vol 15, number 6, 9-19 http://www.val.me.vt.edu/General%20Information/GIslide.html). Active cancellation appears much more complicated than passive mechanical solutions.
The foregoing discussion reflect the current state of the art of which the present inventors are aware. Reference to and discussion of the related prior art is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated technologies disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.